1. Technical Field of the Invention
This invention is concerned with a method and apparatus for measuring desired geophone parameters such as sensitivity, damping and natural frequency.
2. Discussion of Prior Art
A seismometer is a device for detecting and measuring mechanical vibratory motion. Essentially, a seismometer consists of a mass, suspended from a spring secured to a support. A sensing element measures the relative motion between the support and the stationary mass. Seismometers are used in earthquake studies, geophysical exploration for oil, for foundation studies in engineering work, for intruder detection, for monitoring the movements of vibrating machinery and for many other applications. The design parameters of a seismometer are tailored to fit the technology involved.
Various types of sensing elements are known. One common type consists of a spring from which is suspended a coil of wire, that constitutes the mass and moves relative to a magnetic field. In most instruments, the coil and the form upon which it is wound constitute the mass. Relative movement of the coil with respect to the magnetic field generates an electrical signal indicative of the amplitude and frequency of the mechanical vibration. The moving-coil element is widely used with seismometers, also known as geophones, that are used for geophysical exploration for oil. Other types of sensing elemets are based upon magnetostrictive or variable reluctance principles. The sensing elements have in common the ability to convert mechanical input motion into electrical output signals. Conversely, an electrical signal applied to the sensing element will induce a mechanical motion to that element.
For purposes of this disclosure, by way of example but not by way of restriction, moving-coil elements, as used in geophones, will be the subject of the ensuing discussion. A typical moving-coil geophone is disclosed in U.S. Pat. No. 4,159,464, assigned to the assignee of this invention. That patent is incorporated herein by reference as a showing of a typical geophone.
Geophones are manufactured to close tolerances. In the factory, they must be tested to assure uniformity of the output-signal characteristics. In the field, they must be tested to detect a change in parametric tolerances due to use and abuse.
Parameters of interest are sensitivity, damping, natural or reasonant frequency, coil resistance and phase shift.
In one form of test, a geophone to be tested is mounted on a shaking table next to a standard geophone. The two geophones are excited at various frequencies and their outputs are compared. If the output signals match within predetermined limits, the geophone under test is accepted.
Another type of test may be conducted wherein a geophone is driven by a steady-state oscillatory electrical signal. The back EMF of the coil is compared with the driving signal and is expected to meet preset standards.
Both of the above tests are qualitative but not necessarily diagnostic of specific parameters.
A somewhat more analytical test involves applying a mechanical or electrical step function to the geophone being tested. A transient output signal, or signature, results from the step-function output. By use of equations well known in the art, the first four parameters listed above can be calculated. See for example, U.S. Pat. Nos. 4,043,175 and 4,015,202.
All of the above test procedures must be performed upon individual geophones, one at a time. Clearly such tests are slow and cumbersome for production testing in a factory as well as for field testing.
As is well known in geophysical exploration, for purposes of signal enhancement, a plurality of geophones may be employed in an array. The geophones of the array are permanently electrically wired together as a single string. The individual output signals of the repsective geophones, being electrically combined, produce a single composite output signal. A diagnostic test of the individual geophones of such an array is simply not possible using prior-art methods, without violating the electrical integrity of the array as a whole.